Process for preparing aqueous fluoropolymer dispersions and the concentrated aqueous fluoropolymer dispersions produced by such process

ABSTRACT

Fluorine-containing emulsifiers can be removed from fluoropolymer dispersions by multiple cycles of thermal concentration. After repeated cycles of dilution and extraction the fluorine-containing emulsifier can be reduced to a very low level. The resulting dispersion can be formulated into an aqueous dispersion that can be used for coating applications, such as seal, packing, fabric and metal coating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing an aqueous dispersion of polytetrafluoroethylene (PTFE). The present invention also relates to the resulting aqueous dispersions of PTFE produced by such a process.

2. Discussion of the Background

PTFE dispersions can be produced by suspension or emulsion polymerization. A fluorinated emulsifier, ammonium perfluorooctanoate, C₇F₁₅CO₂NH₄ (APFO), is generally used to stabilize the particles as they grow in the autoclave during the polymerization process. Other fluorosurfactants such as straight chain or branched chain C₄-C₁₂-perfluorocarboxylic acid salts (including ammonium perfluorononanoate, APFN), straight chain or branched chain C₄-C₁₂-hydrofluorocarboxylic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane sulfonic acid salts, straight chain or branched chain C₄-C₁₂-hydrofluoroalkane sulfonic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane phosphonic acid salts, and straight chain or branched chain C₄-C₁₂-hydrofluoroalkane phosphonic acid salts may also be used in the process. A typical as-polymerized dispersion contains approximately 5 to 40 wt. % PTFE with a fluorinated emulsifier in water.

U.S. Pat. No. 2,801,962 discloses a method which involves electro-decantation and which may be used to concentrate PTFE aqueous dispersions.

U.S. Pat. No. 3,037,953 discloses the concentration of aqueous colloidal dispersions of PTFE by a thermal process.

U.S. Pat. No. 4,369,266 discloses the preparation of concentrated dispersions of fluorinated polymers by means of ultra-filtration, followed by APFO removal by with an anion exchanger.

Although APFO is a good stabilizer for the polymerization, it is an EPA-restricted fluorosurfactant. Thus, the presence of APFO in the PTFE dispersions raises concerns about its ability to accumulate in the environment and the unknown long-term effects. Therefore, it is desirable to eliminate such compounds from PTFE dispersions. Three methods have been reported to eliminate APFO from aqueous dispersion of PTFE: anion exchange; steam distillation; and multiple electro-decantation (ED) and thermal concentration.

WO 01/79332 A1 discloses a process for removing steam-volatile fluorinated emulsifiers in their free acid form, from aqueous fluoropolymer dispersions. The process comprises adding a nonionic emulsifier to the aqueous fluoropolymer dispersion and, at a pH-value of the aqueous fluoropolymer dispersion below 5, removing steam-volatile fluorinated emulsifier by distillation until the concentration of steam-volatile fluorinated emulsifier in the dispersion reaches the desired value.

WO 03/051988 A2 discloses the removal of fluorine-containing emulsifiers from fluoropolymer dispersions by adding to the dispersion a nonionic emulsifier, removing the fluorine-containing emulsifier by contact with an anion exchanger and separating the dispersion from the anion exchanger.

WO 03/078479 discloses a method for manufacturing an aqueous high-concentration PTFE dispersion with a reduced perfluorocarboxylic acid salt surfactant concentration.

However, there remains a need for an improved process for preparing an aqueous dispersion of polytetrafluoroethylene (PTFE). There also remains a need for the resulting aqueous dispersions of PTFE produced by such a process.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide novel process for preparing concentrated aqueous dispersions of PTFE with reduced content of APFO.

It is another object of the present invention to provide novel concentrated PTFE dispersions, which are prepared by such a process.

These and other objects, which will become apparent during the following detailed description, have been achieved by the inventors' discovery that fluorinated surfactants, such as APFO, may be removed from an aqueous dispersion of PTFE by:

(1) adding a non-perfluorinated emulsifier to an aqueous dispersion of PTFE, wherein said aqueous dispersion comprises a fluorinated surfactant, such as ammonium perfluorooctanoate, to obtain a stabilized dispersion;

(2) heating the stabilized dispersion to a temperature of 40 to 100° C., to obtain a heated dispersion;

(3) maintaining the heated dispersion at that temperature until it separates into an upper supernatant layer that is rich in fluorinated surfactant and a lower concentrated dispersion of PTFE layer with reduced level of fluorinated surfactant;

(4) isolating the lower concentrated dispersion of PTFE from the supernatant upon cooling, to obtain an isolated concentrated dispersion of PTFE; and

(5) diluting the isloated concentrated dispersion of PTFE with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 20 to 50 wt. %, based on the total weight of the concentrated PTFE dispersion.

Typically one to three cycles of steps (1) to (6) are adequate to remove up to 99 wt. % of APFO from the dispersion. Repeat thermal concentration cycles may be carried out until the APFO level is <400 ppm, preferably <300 ppm, preferably <200 ppm, preferably <100 ppm, more preferably <50 and most preferable <30 ppm.

Thus, it has been found that approximately about 40 to 80 wt. % of the APFO may be eliminated from the concentrated dispersion and stays with the supernatant in the process. When the concentrated dispersion is diluted with water to approximately 30 to 35 wt. % and re-concentrated again, another 40 to 80 wt. % of fluorinated emulsifier may be removed through the supernatant. After repeated thermal concentrations the APFO content may be reduced to less than 100 ppm. Depending on the type and the amount of nonionic surfactants used, the APFO concentration may be reduced to below 30 ppm.

In one preferred embodiment, the process further involves:

(1a) adding a base to the stabilized dispersion, to obtain a basified dispersion, preferably having a pH>9.

In this case, step (2) involves heating the basified dispersion.

In another preferred embodiment, the process further involves:

(1b) adding an electrolyte to the stabilized dispersion, to obtain an electrolyte-containing dispersion.

In this case, step (2) involves heating the electrolyte-containing dispersion.

The dispersions of PTFE obtained from the present process can be formulated into an aqueous dispersion product to be used for, e.g., coating applications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, in a first embodiment, the present invention provides novel processes for preparing an aqueous dispersion of PTFE by:

(1) adding a non-perfluorinated emulsifier to an aqueous dispersion of PTFE, wherein said aqueous dispersion comprises a fluorinated surfactant, such as ammonium perfluorooctanoate, to obtain a stabilized dispersion;

(2) heating the stabilized dispersion to a temperature of 40 to 100° C., to obtain a heated dispersion;

(3) maintaining the heated dispersion at that temperature until it separates into an upper supernatant layer that is rich in fluorinated surfactant and a lower concentrated dispersion of PTFE layer with reduced level of fluorinated surfactant;

(4) isolating the lower concentrated dispersion of PTFE from the supernatant upon cooling, to obtain an isolated concentrated dispersion of PTFE; and

(5) diluting the isolated concentrated dispersion of PTFE with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 20 to 50 wt. % , based on the total weight of the concentrated PTFE dispersion.

The starting dispersion of PTFE used in this process can be any PTFE dispersion produced by conventional dispersion polymerization such as homo polymerization or trace comonomer polymerization of tetrafluoroethylene (TFE). In a preferred embodiment, the starting dispersion of PTFE is an aqueous autoclave dispersion.

The surfactant used in the polymerization of TFE and contained in the starting PTFE dispersion is a fluorinated emulsifier, such as ammonium perfluorooctanoate, C₇F₁₅CO₂NH₄ (APFO). Other fluorosurfactants such as straight chain or branched chain C₄-C₁₂-perfluorocarboxylic acid salts (including ammonium perfluorononanoate, APFN), straight chain or branched chain C₄-C₁₂-hydrofluorocarboxylic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane sulfonic acid salts, straight chain or branched chain C₄-C₁₂-hydrofluoroalkane sulfonic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane phosphonic acid salts, and straight chain or branched chain C₄-C₁₂-hydrofluoroalkane phosphonic acid salts may also used in the process and may also be present in the starting PTFE dispersion.

The molecular weight of PTFE in these dispersions is typically reported in terms of standard specific gravity (SSG) that can be measured by the ASTM D-4895, and it ranges from 1.30 to 2.30, preferably 1.90 to 2.30.

The PTFE contained in the aqueous dispersion may be a homopolymer of tetrafluoroethylene (TFE). Alternatively, the PTFE in the aqueous dispersion may contain one or more comonomers used in the co-polymerization of TFE, such as hexafluoropropene (HFP), perfluoro-(n-propyl-vinyl)-ether (PPVE), perfluorohexyl vinylether (PHVE), chloro-trifluoroehtylene (CTFE), and Cytop. In the case of a PTFE copolymer, the PTFE copolymer will typically contain 70 to 99 wt. %, preferably 80 to 99 wt. %, of monomeric units derived from TFE. In a particularly preferred embodiment, the PTFE is a homopolymer of TFE.

The starting aqueous dispersion of PTFE typically contains 0.1 to 0.5 wt. % of fluorinated surfactant, e.g., APFO, based on the weight of PTFE. The PTFE is typically present in an amount of between 5 to 40 wt. %, based on the total weight of the dispersion. The water content ranges from 60 to 95 wt. %, based on the total weight of the dispersion. The starting dispersion typically has a pH value of between 3.0 to 5.0.

The average primary particle size of the PTFE in the dispersion ranges from 0.1 to 0.5 microns, more typically between 0.2 to 0.4 microns.

1. Stabilizing the Starting Dispersion with a Non-Fluorinated Emulsifier:

In the first step, the starting aqueous dispersion of PTFE is stabilized with a non-fluorinated emulsifier so that it can be transferred from vessel to vessel without coagulation. Suitable emulsifiers are disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, 4^(th) Ed., Wiley, New York, vol. 23, pp. 478-541, 1997, and in Milton Rosen, Surfactants and Interfacial Phenomena, 2^(nd) Ed. Wiley, New York 1989.

The preferred class of emulsifiers is nonionic surfactants, although both anionic and cationic surfactant can be added for special applications in which coagulation is a necessary step in the process. Specific examples of nonionic surfactants include polyoxyethylenes such as alcohol ethoxylates and alkylphenol ethoxylates; carboxylic esters such as glycerol esters, polyoxyethylene esters; anhydrosorbitol esters, such as ethoxylated anhydrosorbitol esters; natural ethoxylated fats, oils, and waxes; glycol esters of fatty acids; alkyl polyglycosides; carboxylic amides, such as diethanolamine condensates, monoalkanolamine condensates, and polyoxyethylene fatty acid amides; fatty acid glucamides; polyalkylene oxide block polymers; and poly(oxyethylene-co-oxypropylene)nonionic surfactants. Specific examples of suitable emulsifiers include Triton N-11, Synperonic OP11EO, Synperonic 91/6, Tergitol 15-S-12, Tergitol 15-S-9, Tergitol 15-S-7, Newcol 1310, and Dispanol TOC. Preferred emulsifiers include Newcol 1308FA(90), Ethox 4031 Mod 38, Ethox 4031 Mod 64, Synperonic OP10EO, and Triton X-100. A particularly preferred emulsifier is Ethox 4031.

Examples of suitable anionic surfactants include sodium lauryl sulfate and ammonium lauryl sulfate. Other suitable anionic surfactants are carboxylic acid salts such as sodium and potassium salts of straight-chain fatty acids; sulfuric acid ester salts such as sulfated tallow alcohols or sulfated synthetic alcohols from linear olefins; sulfated polyoxyethylenated straight-chain alcohols with structure of R(OC₂H₄)_(x)SO₄ ⁻M⁺, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; and sulfated triglyceride oils.

Suitable cationic surfactants include Ethomeen T/12, Ethomeen T/15, Ethomeen 18/15, Ethomeen 18/25 and Ethomeen 18/60. Other suitable cationic surfactants are long-chain amines and their salts. These are primary amines derived from animal and vegetable fatty acids and tall oil, synthetic C₁₂-C₁₈ primary, secondary, or tertiary amines; diamines and polyamines and their salts with structure of (RCONHCH₂CH₂)₂NH, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; polyoxyethylenated (POE) long-chain amines with structure of RN[(CH₂CH₂O)_(x)H]₂, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; quatemized polyoxyethylenated long-chain amines with structure of RN(CH₃)[(C₂H₄O)_(x)H]₂ ⁺Cl⁻, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; and amine oxides such as—N-C₈-C₂₀-alkyldimethylamine oxides.

The amount of non-fluorinated emulsifier to be added is from 5 to 25 wt. %, preferably 15 to 20 wt. %, based on the weight of the PTFE in the dispersion. The amount of non-fluorinated emulsifier added will influence the amount of fluorinated surfactant, e.g., APFO, that is removed from the dispersion. The amount of non-fluorinated emulsifier added also affects the rate and the efficiency of concentration.

The cloud point of the non-fluorinated emulsifier also affects the rate of concentration and the amount needed in the process. The lower the cloud point, the less non-fluorinated emulsifier will be needed and the lower the temperature required to induce phase separation. Since APFO has a tendency to stay with the nonionic surfactant, a higher level of non-fluorinated emulsifier helps to extract more APFO in the process.

The cloud point of a surfactant can be determined by heating a 1 wt. % solution in water to a temperature where it turns completely turbid. This information is usually provided by the manufacturers on the certificate of analysis. Good results have been achieved by adding 15 to 18 wt. % of Ethox 4031 or Triton X-100, based on the weight of PTFE in the dispersion.

The thermal concentration can be carried out in a stainless steel vessel, which is equipped with an agitator and a temperature control device.

1a. Basifying the Stabilized Dispersion:

In a preferred embodiment, after the addition of the non-fluorinated emulsifier, the pH of the stabilized dispersion is raised by the addition of a base. The pH of the stabilized dispersion is typically raised to a value of 8 to 10, preferably 9 to 10. Typically, either ammonium hydroxide (30 wt. % aqueous solution) or ammonium carbonate (10 wt. % aqueous solution) can be used for the basification. Ammonium hydroxide is preferred due to its ease of handling and its effectiveness in rate of concentration, and also because it can be easily burned off without any residue during the sintering cycle of glass-cloth and metal coating, and it is universally accepted as a pH modifier. Typically 0.1 to 1.0 wt. %, more preferably 0.1 to 0.3 wt. %, of ammonium hydroxide (30 wt. % aqueous solution), based on the total weight of the dispersion will be sufficient to raise the pH above 9.

Other bases, such as sodium hydroxide and sodium carbonate, can also induce thermal concentration. But their use may be limited to applications where metal ions are unimportant in the final product.

1b. Adding an Electrolyte to the Stabilized Dispersion:

In another preferred embodiment, after the addition of the non-fluorinated emulsifier, an electrolyte is added to the stabilized dispersion. In thermal concentration process which includes step (1a), a base is added to induce rapid phase separation. It has also been found that addition of an electrolyte can also facilitate phase separation.

Specific examples of suitable electrolytes include LiCl, NaCl, KCl, magnesium chloride, barium chloride, ammonium chloride, LiBr, NaBr, KBr, magnesium bromide, barium bromide, ammonium bromide, LiI, NaI, KI, magnesium iodide, barium iodide, ammonium iodide, lithium nitrate, sodium nitrate, potassium nitrate, magnesium nitrate, barium nitrate, ammonium nitrate, lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, barium sulfate, ammonium sulfate, lithium sulfite, sodium sulfite, potassium sulfite, magnesium sulfite, barium sulfite, ammonium sulfite, lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, barium carbonate, etc. Typically, the electrolyte is added to the stabilized dispersion in an amount of 0.01 to 0.5 wt. %, preferably 0.05 to 0.15 wt. %, based on the total weight of the PTFE in the stabilized dispersion.

However, since the presence of metal ions may cause adverse effects in the final product when used in applications, such as electronics applications, it is often preferred to omit the addition of an electrolyte in such cases. Instead, the addition of a base as described in 1a may be used. Of course, step (1b) may be used in conjunction with step (1b).

2. Heating the Stabilized Dispersion:

After addition of the non-fluorinated emulsifier (and optionally, the adjustment of pH), the dispersion is then heated. Preferably, the dispersion is slowly heated, with mild agitation, to keep the temperature uniform throughout the dispersion, especially when the process is carried out on a large scale production. The agitation is stopped once phase separation starts, at approximately the cloud point of the surfactant. The temperature of the dispersion is raised to at least 5 to 20° C., preferably 10 to 15° C., above the cloud point of the non-fluorinated emulsifier, and the dispersion is maintained at that temperature long enough to complete the sedimentation of the dispersion.

3. Phase Separation:

When the temperature of the dispersion is higher than the cloud point of the non-fluorinated emulsifier, PTFE particles start to settle out to the bottom to form a dense layer of latex, while leaving a clear layer of supernatant on top. The rate of sedimentation is dependent on the amount of non-fluorinated emulsifier used and the cloud point of the non-fluorinated emulsifier. These effects are shown in the examples. The concentrated latex typically contains about 50 to 75 wt. % of PTFE, based on the total weight of the latex. A higher level of non-fluorinated emulsifier will promote a faster rate of sedimentation and higher solids level in the final product latex and vice versa. About 40 to 80 wt. % of the fluorinated surfactant, e.g., APFO, is removed from the latex and stays in the supernatant layer.

4. Isolating the Lower Concentrated Dispersion of PTFE:

Once the dispersion is cooled to room temperature, the concentrated latex is isolated from the fluorinated surfactant-rich supernatant. In a preferred embodiment, the concentrated latex is isolated by decanting or draining it from the bottom and transferring it to separate container. The top liquid phase, which is enriched with non-fluorinated emulsifier and fluorinated surfactant, e.g., APFO, may then be removed and saved in a separate vessel for recycling or disposal after treatment. Isolation of the two layers is tricky, since a concentration gradient exists throughout the enriched portion of the dispersion, with a higher solids at the bottom and a lower solids near the interface. Therefore removal of the bottom layer usually stops at about 98 wt. % of the concentrated latex to avoid mixing with the fluorinated surfactant-rich supernatant layer near the interface. The 2 wt. % of the latex near the interface may be saved for future thermal concentration or reworked into different products.

5. Diluting the Isolated Concentrated Dispersion of PTFE:

After being isolated, the concentrated latex is then diluted with purified water to approximately 15 to 45 wt. %, preferably 30 to 35 wt. % PTFE, by weight based on the total weight of the dispersion. Any emulsifier lost to the supernatant may be replenished and additional base may be added. Then, the whole thermal concentration process may be repeated as shown in the above steps (1-5). The fluorinated surfactant, e.g., APFO, and non-fluorinated emulsifier left in the supernatant can be recovered by filtration (see, U.S. Pat. No. 4,369,266) or extraction (see, U.S. Pat. No. 4,639,337) methods. The purified emulsifier can be reused for subsequent concentrations. Typically approximately 40 to 80 wt. % of the fluorinated surfactant, e.g., APFO, in the dispersion is removed with each cycle.

In a preferred embodiment, steps (1) through (5), with or without steps (1a) and/or (1b), are repeated at least once, preferably at least twice, more preferably at least three times, even more preferably at least four times, even more preferably at least five times. In a particularly preferred embodiment, steps (1) through (5), with or without steps (1a) and/or (1b), are repeated 1 to 4 times.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

EXAMPLES

In the following examples, all the APFO levels were measured by an independent laboratory, Exygen Research using High Pressure Liquid Chromatography (HPLC). The solids content of a dispersion is based on the total weight of the dispersion, and the emulsifier content is based on the weight of the solids (PTFE) of the dispersion. The APFO level is expressed in ppm (parts per million) of the total weight of sample. All temperatures are in degrees Centigrade.

The following test methods were used to evaluate the coating performance of a dispersion from the process:

The amount of APFO was measured by Liquid Chromatography conducted by Exygen Research Laboratory.

Viscosity transition temperature was measured with a Brookfield Viscometer and was the temperature at which dispersion viscosity shows a step change.

Particle size was measured with a Coulter Particle Size analyzer model LS230.

Shear stability was measured by circulating the dispersion in a beaker with a peristaltic pump through Tygon tubing for a certain period of time. The amount of coagulum collected at the end of the test was dried and weighed. The weight of the coagulum was divided by the total weight of the dispersion and expressed as a percentage to give the value of stability.

Settling stability was measured by centrifuging a known quantity of dispersion for a period of time. At the end of the test, the latex on top of the centrifuge tube was discarded leaving the coagulum at the bottom. The net weight of the coagulum was measured and was divided by the total weight of the dispersion and expressed as a percentage to give the value of stability.

Critical film thickness was a measure of the maximum coating thickness on a test panel at which the surface was crack free.

Rewetting was a measure of the wetting ability of the coating. It was measured by dipping a test panel in a diluted dispersion and observing its ability to wet the surface. The lower the solids level to wet the panel the better is its performance.

Example 1

Four samples of PTFE dispersions containing 19.3 wt. % of PTFE, 754 ppm of APFO (3907 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 were processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Triton X-100. 0.1     wt. % of NH₄OH (30 wt. % aqueous solution) was added to adjust the     pH. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 83° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 75° C. -   5. The temperature was maintained between 75 and 80° C. until phase     separation was complete. -   6. The concentrated latex contained 66.9 wt. % of PTFE, 2.6 wt. %     Triton X-100 and 450 ppm of APFO (673 ppm of APFO based on the     weight of PTFE). The APFO reduction from 3907 to 450 ppm (based on     PTFE) was 83% after one cycle of thermal concentration. -   7. The supernatant was drained into a separate container for future     recycling. -   8. The concentrated latex was diluted to 33 wt. % PTFE with purified     water and re-concentrated according to step 1 to 7 as shown above     except that the total surfactant was reduced to 10 wt. %. NH₄OH were     added to adjust the pH. -   9. The concentrated latex contained 63.5 wt. % PTFE, 3.1 wt. %     Triton X-100 and 209 ppm (329 ppm by weight of PTFE). The APFO     reduction from 673 to 329 ppm (based on PTFE) was 51 % on the second     cycle of thermal concentration. -   10. The latex was diluted to 33 wt. % PTFE with purified water and     re-concentrated for the 3^(rd) time as described above. -   11. The concentrated latex contained 65.9 wt. % PTFE, 5.1 wt. %     Triton X-100 and 127 ppm (193 ppm by weight of PTFE). The APFO     reduction from 329 to 193 ppm was 41% on the 3^(rd) cycle of thermal     concentration. The overall APFO reduction from 3907 to 193 ppm     (based on PTFE) was about 95%. -   12. The concentrated latex was used for coating application.

Example 2

An aqueous dispersion of PTFE with trace commoner PPVE containing 18.8 wt. % by weight of modified PTFE, 210 ppm of APFO (1117 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Newcol 1308 FA(90).     NH₄OH (30 wt. % aqueous solution) was added to adjust the pH. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 54° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 50° C. -   5. The temperature was maintained between 50 and 52° C. until phase     separation was complete. -   6. The concentrated latex contained 64.3 wt. % PTFE, 3.1 wt. %     Newcol 1308 FA(90) and 188 ppm of APFO (292 ppm of APFO based on the     weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (1117 to 292 ppm on PTFE) was 74%. -   9. The concentrated latex was used for metal coating application.

Example 3

An aqueous dispersion of PTFE with trace commoner PPVE containing 20.5 wt. % by weight of modified PTFE, 66 ppm of APFO (322 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Triton X-100. 0.25     ml of NH₄OH (30 wt. % aqueous solution) was added to adjust the pH. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 80° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 72° C. -   5. The temperature was maintained between 70 and 75° C. until phase     separation was complete. -   6. The concentrated latex contained 71.8 wt. % PTFE, 4 wt. % Triton     X-100 and 108 ppm of APFO (150 ppm of APFO by weight of PTFE). -   7. The supematant was drained into a separate container for future     recycling. -   8. The reduction of APFO (322 to 150 ppm on PTFE) was 53%. -   9. The concentrated latex was used for metal coating application.

Example 4

An aqueous dispersion of PTFE with trace commoner PPVE containing 8.4 wt. % by weight of modified PTFE, 20 ppm of APFO (238 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 20 wt. % of Triton X-100. 0.25     ml of NH₄OH (30 wt. % aqueous solution) was added to adjust the pH. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 78° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 74° C. -   5. The temperature was maintained between 70 and 75° C. until phase     separation was complete. -   6. Concentrated latex contained 21.1% PTFE, 9.5 wt. % Triton X-100     and 31 ppm of APFO (147 ppm of APFO by weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (238 to 147 ppm on PTFE) was 38% after one     cycle of thermal concentration. -   9. The concentrated latex was re-concentrated according to step 1 to     7 as shown above. Additional surfactant and NH₄OH were added to keep     the levels constant. -   10. The concentrated latex contained 70 wt. % PTFE, 4.4 wt. % Triton     X-100 and 63 ppm (90 ppm by weight of PTFE). The APFO reduction from     147 to 90 ppm (based on PTFE) was 39% on the second cycle of thermal     concentration. The overall APFO reduction from 238 to 90 ppm (based     on PTFE) was about 62%. -   11. The concentrated latex was used for metal coating application.

Example 5

A PTFE dispersion containing 26.6 wt. % by weight of PTFE, which had a higher average molecular weight than previous examples, 934 ppm of APFO (3511 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Ethox 4031 Mod 38.     NH₄OH (30 wt. % aqueous solution) was added to adjust the pH. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 50° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 50° C. -   5. The temperature was maintained between 48 and 50° C. until phase     separation was complete. -   6. The concentrated latex contained 68.4 wt. % PTFE, 3.1 wt. % Ethox     4031 Mod 38 and 482 ppm of APFO (705 ppm of APFO based on the weight     of PTFE). The APFO reduction from 3511 to 705 ppm (based on PTFE)     was 80% after one cycle of thermal concentration. -   7. The supernatant was drained into a separate container for future     recycling. -   8. The concentrated latex was diluted to 36 wt. % PTFE with purified     water and re-concentrated according to step 1 to 7 as shown above.     Additional surfactant and NH₄OH were added to keep the levels     constant. -   9. The concentrated latex containing 70.8 wt. % PTFE, 3.1 wt. %     Ethox 4031 Mod 38 and 107 ppm (151 ppm by weight of PTFE) was     collected. The APFO reduction from 705 to 151 ppm (based on PTFE)     was 79% on the second cycle of thermal concentration. -   10. The latex was diluted to 33.2 wt. % PTFE with purified water and     re-concentrated for the 3^(rd) time as described above. -   11. The concentrated latex contained 75.7 wt. % PTFE, 2.6 wt. %     Ethox 4031 Mod 38 and 44 ppm (58 ppm by weight of PTFE). The APFO     reduction from 151 to 58 ppm was 62% on the 3^(rd) cycle of thermal     concentration. The overall APFO reduction from 3511 to 58 ppm (based     on PTFE) was about 98%. -   12. The concentrated latex was used for coagulation application.

Example 6

A PTFE dispersion containing 30 wt. % by weight of PTFE, which had a lower average molecular weight than previous examples, 732 ppm of APFO (2440 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Newcol 1308 FA(90)     and basified with NH₄OH (30 wt. % aqueous solution). -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 55° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 50° C. -   5. The temperature was maintained between 50 and 55° C. until phase     separation was complete. -   6. The concentrated latex contained 65.8 wt. % PTFE, 3.5 wt. %     Newcol 1308 FA(90) and 320 ppm of APFO (486 ppm of APFO based on the     weight of PTFE). The APFO reduction from 2440 to 486 ppm (based on     PTFE) was 80% after one cycle of thermal concentration. -   7. The supernatant was drained into a separate container for future     recycling. -   8. The concentrated latex was diluted to 35 wt. % PTFE with purified     water and re-concentrated according to step 1 to 7 as shown above.     Additional emulsifier and base were added to keep the levels     constant. -   9. The concentrated latex contained 65 wt. % PTFE, 3.5 wt. % Newcol     1308 FA(90) and 140 ppm (215 ppm by weight of PTFE). The APFO     reduction from 486 to 215 ppm (based on PTFE) was 56% on the second     cycle of thermal concentration. -   10. The latex was diluted into 35% PTFE with purified water and     re-concentrated for the 3^(rd) time. -   11. The concentrated latex contained 68.6 wt. % PTFE, 3.5 wt. %     Newcol 1308 FA(90) and 29 ppm (42 ppm by weight of PTFE). The APFO     reduction from 215 to 42 ppm was 80% on the 3^(rd) cycle of thermal     concentration. The overall APFO reduction from 2440 to 29 ppm (based     on PTFE) was about 99%. -   12. The concentrated latex was formulated and used for coating     application.

The performance compared favorably with the control and the results are summarized in Table 1. TABLE 1 Comparison of property of the low APFO with typical PTFE aqueous dispersion. Low APFO PTFE Property Aqueous dispersion Aqueous dispersion ID KFA64-1 Control PTFE 60.4 wt. % 60.5 wt. % Stabilizer 6.3 wt. % 6.6 wt. % APFO (on total weight) 29 ppm 1430 ppm PH 9.9 10 Viscosity (25°, 60 rpm) 24 cps 25 cps Viscosity Transition 32.2° C. 28.3° C. Temperature Particle size 0.269 μm 0.268 μm Shear stability. 0.68% 0.81% % coagulum Settling stability. 18.5% 17.5% % coagulum Critical Film Thickness 23 ± 1 μm 24 ± 1 μm Rewet, passed:   30%   30% Failed:   25%   25%

Example 7

A raw PTFE dispersion containing 26 wt. % by weight of PTFE with the same average molecular weight as Example 1, 700 ppm of APFO (2692 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 10 wt. % of Newcol 1308 FA(90).     0.08 wt. % of Ammonium Lauryl Sulfate (30 wt. % aqueous solution)     and NH₄OH (30 wt. % aqueous solution) were added. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 55° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 50° C. -   5. The temperature was maintained between 50 and 52° C. until phase     separation was complete. -   6. The concentrated latex contained 63.3 wt. % PTFE, 2.9 wt. %     Newcol 1308 FA(90)+ALS and 342 ppm of APFO (540 ppm of APFO based on     the weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (2692 to 540 ppm on PTFE) was 80%.

Example 8

A raw PTFE dispersion containing 26.6 wt. % by weight of PTFE, which has the same average molecular weight of Example 1, 700 ppm of APFO (2632 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 10 wt. % of Ethomeen T/15 and     basified with NH₄OH (30 wt. % aqueous solution). -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 45° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 43° C. -   5. The temperature was maintained between 43 and 45° C. until phase     separation was complete. -   6. The concentrated latex contained 37 wt. % PTFE, 3 wt. % Ethomeen     T/15 and 289 ppm of APFO (781 ppm of APFO based on the weight of     PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (2632 to 781 ppm on PTFE) was 70%.

Example 9

A raw PTFE dispersion containing 26.6 wt. % by weight of PTFE, which has the same average molecular weight as Example 1, 700 ppm of APFO (2632 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 10 wt. % of Newcol 1308 FA(90)     and 10 wt. % of Ethomeen T/15 and basified with NH₄OH (30 wt. %     aqueous solution). -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 55° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 50° C. -   5. The temperature was maintained between 50 and 55° C. until phase     separation was complete. -   6. The concentrated latex contained 48 wt. % PTFE, 4.3 wt. %     stabilizer (Ethomeen T/15 and Newcol), and 380 ppm of APFO (792 ppm     of APFO based on the weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (2632 to 792 ppm on PTFE) was 70%.

Example 10

A raw PTFE dispersion containing 26.6 wt. % by weight of PTFE, which has the same average molecular weight as Example 1, 710 ppm of APFO (2669 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Ethox 4031 and 1     wt. % of Ethomeen T/15. -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 60° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 57° C. -   5. The temperature was maintained between 56 and 58° C. until phase     separation was complete. -   6. The concentrated latex contained 63 wt. % PTFE, 4.7 wt. %     stabilizer (Ethomeen T/15 and Ethox 4031), and 1119 ppm of APFO     (1776 ppm of APFO based on the weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (2669 to 1776 ppm on PTFE) was 33%.

Example 11

A raw PTFE dispersion containing 26.6 wt. % by weight of PTFE, which has the same average molecular weight as Example 1, 710 ppm of APFO (2669 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was stabilized with 18 wt. % of Ethox 4031 and 1     wt. % of Ethomeen T/15 and basified with NH₄OH (30% aqueous     solution). -   2. It was then transferred into a lab Thermal Concentrator, which     was connected to a circulating temperature control bath with maximum     temperature set at 60° C. -   3. The dispersion was slowly heated up from room temperature with no     agitation. -   4. Heating continued until temperature reached about 57° C. -   5. The temperature was maintained between 56 and 58° C. until phase     separation was complete. -   6. The concentrated latex contained 68.5 wt. % PTFE, 3.5 wt. %     stabilizer (Ethomeen T/15 and Ethox 4031), and 552 ppm of APFO (806     ppm of APFO based on the weight of PTFE). -   7. The supernatant was drained into a separate container for future     recycling. -   8. The reduction of APFO (2669 to 806 ppm on PTFE) was 70%.

Example 12

A 252 gals of raw PTFE dispersion containing 28.7 wt. % by weight of PTFE with the same average molecular weight as Example 1, 614 ppm of APFO (2139 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was first stabilized with 2.5 wt. % of Triton X-100     and stored in a stock tank. -   2. It was then transferred into the Pilot Thermal Concentrator. -   3. The dispersion was slowly heated up from room temperature with     mild agitation. -   4. At 45° C. a mixture of 18 wt. % Triton X-100 and NH₄OH (30 wt. %     aqueous solution) was added to the dispersion. -   5. When temperature reached 60° C. the agitator was stopped to allow     phase separation without disturbance. -   6. Heating continued until temperature reached 75° C. -   7. The temperature was maintained at 75° C. until phase separation     was complete. -   8. About 85 gal of latex containing 62.6 wt. % PTFE, 5.1 wt. %     Triton X-100 and 647 ppm of APFO (1033 ppm of APFO based on the     weight of PTFE) was collected. The APFO reduction from 2139 to 1033     ppm (based on PTFE) was 52% after one cycle of thermal     concentration. -   9. The supernatant was drained into a separate container for future     recycling. -   10. The 85 gal of latex was diluted to 32.7 wt. % PTFE with purified     water and re-concentrated according to step 1 to 7 as shown above.     Additional surfactant and base were added. -   11. About 60 gal of latex containing 64.1 wt. % PTFE, 4.2 wt. %     Triton X-100 and 150 ppm (234 ppm by weight of PTFE) was collected.     The APFO reduction from 1033 to 234 ppm (based on PTFE) was 77% on     the second cycle of thermal concentration. -   12. The latex was diluted to 36.8 wt. % PTFE with purified water and     re-concentrated again for the 3^(rd) time. -   13. About 50 gal of latex containing 72 wt. % PTFE, 4 wt. % Triton     X-100 and 73 ppm (101 ppm by weight of PTFE) was collected. The APFO     reduction from 234 to 73 ppm was 69% on the 3^(rd) cycle of thermal     concentration. The overall APFO reduction from 2139 to 101 ppm     (based on PTFE) was about 95%. -   14. The latex was formulated into a coating grade aqueous dispersion     and its performance compared favorably with the control.

The results are summarized in Table 2. TABLE 2 Comparison of property of low APFO with typical PTFE aqueous dispersion. Low APFO PTFE Property Aqueous dispersion Aqueous dispersion ID PC61-69 Control PTFE 59.0 wt. % 60.5 wt. % Stabilizer 8.0 wt. % 8.0 wt. % APFO (on total weight) 73 ppm 1292 ppm pH 10 10 Viscosity (25°, 60 rpm) 28.3 cps 29.4 cps Viscosity Transition 28.6° C. 33° C. Temperature Particle size 0.267 μm 0.268 μm Shear stability. 0.42% 0.36% % coagulum Settling stability. 17.5% 16.7% % coagulum Critical Film Thickness 26 ± 1 μm 24 ± 1 μm Rewet, passed:   30%   20% Failed:   25%   15%

Example 13

A 455 gals of PTFE dispersion containing 26.6 wt. % by weight of PTFE with the same average molecular weight as Example 1, 691 ppm of APFO (2598 ppm of APFO based on the weight of PTFE), and having a pH of 3.5 was processed as follows:

-   1. The raw latex was first stabilized with 2.5 wt. % of Ethox 4031     and stored in a stock tank. -   2. It was then transferred into the Pilot Thermal Concentrator. -   3. The dispersion was slowly heated up from room temperature with     mild agitation. -   4. At 30° C. a mixture of 18 wt. % of Ethox 4031 and NH₄OH (30 wt. %     aqueous solution) was added to the dispersion. -   5. When temperature reached 47° C. the agitator was stopped to allow     phase separation without disturbance. -   6. Heating continued until temperature reached 55° C. -   7. The temperature was maintained between 60 and 70° C. until phase     separation was complete. -   8. About 235 gal of latex containing 47 wt. % PTFE, 3.6 wt. % Ethox     4031 and 261 ppm of APFO (555 ppm of APFO based on the weight of     PTFE) was collected. The APFO reduction from 2598 to 555 ppm (based     on PTFE) was 79% after one cycle of thermal concentration. -   9. The supernatant was drained into a separate container for future     recycling. -   10. The 235 gal of latex was diluted to 32 wt. % PTFE with purified     water and re-concentrated according to step 1 to 7 as shown above.     Additional surfactant and base were added. -   11. About 140 gal of latex containing 58.9 wt. % PTFE, 4 wt. % Ethox     4031 and 162 ppm (275 ppm by weight of PTFE) was collected. The APFO     reduction from 555 to 275 ppm (based on PTFE) was 50% on the second     cycle of thermal concentration. -   12. The latex was diluted to 35 wt. % PTFE with purified water and     re-concentrated again for the 3^(rd) time. -   13. About 94 gal of latex containing 65.4 wt. % PTFE, 2.9 wt. %     Ethox 4031 and 55 ppm (84 ppm by weight of PTFE) and 27 gal of latex     containing 51 wt. % PTFE were collected. The lower solids latex was     be saved for future use. The APFO reduction from 275 to 84 ppm was     69% on the 3^(rd) cycle of thermal concentration. The overall APFO     reduction from 2598 to 84 ppm (based on PTFE) was about 97%. -   14. The latex was formulated into a coating grade aqueous dispersion     and the performance compared favorably with the control.

The results are summarized in Table 3. TABLE 3 Comparison of property of low APFO with typical PTFE aqueous dispersion. Low APFO PTFE Property Aqueous dispersion Aqueous dispersion ID KFA65-46 Control PTFE 60.4 wt. % 60.5 wt. % Stabilizer 6.3 wt. % 6.6 wt. % APFO (on total weight) 63 ppm 1430 ppm pH 9.9 10 Viscosity (25°, 60 rpm) 27.5 cps 25 cps Viscosity Transition 35.4° C. 28.3° C. Temperature Particle size 0.249 μm 0.268 μm Shear stability. 0.96% 0.81% % coagulum Settling stability. 16.1% 17.5% % coagulum Critical Film Thickness 23 ± 1 μm 24 ± 1 μm Rewet, passed:   35%   30% Failed:   30%   25%

The results of all the examples are summarized in Table 4. It was found that 2 to 3 cycles of thermal concentration was adequate to reduce APFO to less than 100 ppm. About 99% of APFO could be removed from latex in 3 cycles of thermal concentration. TABLE 4 Summary of the APFO elimination from PTFE dispersions by thermal concentration. Surfactant in Initial Final Surfactant, (% on final conc. Number of Initial APFO Final APFO APFO Polymer PTFE PTFE PTFE) added each dispersion thermal (on total weight, (on total reduction Example Type (wt. %) (wt. %) cycle (wt. %) conc. cycles ppm) weight, ppm) (%)  1 PTFE 19.3 65.9 18% TX100 5.1% 3 754 127 95 Homo- (1^(st) cycle) Triton (3907 on PTFE) (193 on PTFE) polymer 10% TX100 X-100 (2^(nd) cycle) 12% TX100 (3^(rd) cycle)  2 PTFE 18.8 64.3 18% Newcol 1308 3.1% Newcol 1 210 188 74 Trace FA(90) 1308 FA(90) (1117 on PTFE) (292 on PTFE) PPVE  3 PTFE 20.5 71.8 18%   4% 1  66 108 53 Trace Triton Triton (322 on PTFE) (150 on PTFE) PPVE X-100 X-100  4 PTFE  8.4 70   20%  20% 2  20  63 62 Trace Triton Triton (238 on PTFE) (90 on PTFE) PPVE X-100 X-100  5 PTFE 26.6 75.7 18% 2.6% 3 934  44 98 Homo- Ethox 4031 Ethox (3511 on PTFE) (58 on PTFE) polymer Mod 38 4031 Mod 38  6 PTFE 30   68.6 18% 3.5% 3 732  29 99 Homo- Newcol Newcol (2440 on PTFE) (42 on PTFE) polymer 1308 1308 FA(90) FA(90)  7 PTFE 26   63.3 10% 2.9% 1 700 342 80 Homo- Newcol + Newcol + (2692 on PTFE) (540 on PTFE) polymer 0.08% ALS ALS  8 PTFE 26.6 37   10%   3% 1 700 289 70 Homo- Ethomeen Ethomeen (2632 on PTFE) (781 on PTFE) polymer T/15 T/15  9 PTFE 26.6 48   10% 4.3% 1 700 380 70 Homo- Newcol + (Newcol + (2632 on PTFE) (792 on PTFE) polymer 10% Ethomeen Ethomeen T/15) T/15 10 PTFE 26.6 63   18% 4.7% 1 710 1119  33 Homo- (Ethox (Ethox (2669 on PTFE) (1776 on PTFE) polymer 4031 + 4031 + 1% Ethomeen Ethomeen T/15) T/15) 11 PTFE 26.6 68.5 18% 3.5% 1 690 552 70 Homo- (Ethox (Ethox (2669 on PTFE) (806 on PTFE) polymer 4031 + 4031 + 1% Ethomeen Ethomeen T/15) T/15) 12 PTFE 28.7 72   18%   4% 3 614  73 95 Homo- Triton Triton (2139 on PTFE) (101 on PTFE) polymer X-100 X-100 13 PTFE 26.6 65.4 18% 2.9% 3 691  55 97 Homo- Ethox Ethox (2598 on PTFE) (84 on PTFE) polymer 4031 4031

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

All patents and other references mentioned above are incorporated in full herein by this reference, the same as if set forth at length. 

1. A process for preparing a concentrated aqueous dispersion of PTFE, which comprises: (1) adding a non-fluorinated emulsifier to a starting aqueous dispersion of PTFE, wherein said starting aqueous dispersion of PTFE comprises a fluorinated surfactant, to obtain a stabilized dispersion; (2) heating said stabilized dispersion to a temperature of 40 to 100° C., to obtain a heated dispersion; (3) maintaining said heated dispersion at said temperature until it separates into an upper supernatant layer that is rich in said fluorinated surfactant and a lower concentrated dispersion of PTFE layer with reduced level of said fluorinated surfactant; (4) isolating said lower concentrated dispersion of PTFE from said supernatant upon cooling, to obtain an isolated concentrated dispersion of PTFE; and (5) diluting said isloated concentrated dispersion of PTFE with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 20 to 50 wt. %, based on the total weight of said concentrated PTFE dispersion.
 2. The process of claim 1, further comprising: (6) subjecting said concentrated aqueous PTFE dispersion to a repetition of steps (1) to (5).
 3. The process of claim 2, wherein said repetition of steps (1) to (6) is carried out from 1 to 4 times.
 4. The process of claim 1, further comprising: (7) collecting said upper supernatant layer for surfactant recycling.
 5. The process of claim 1, wherein said lower concentrated dispersion of PTFE is diluted with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 30 to 36 wt. %, based on the total weight of said concentrated PTFE dispersion.
 6. The process of claim 1, which further comprises: (1a) adding a base to said stabilized dispersion, to obtain a basified dispersion, prior to said heating (2).
 7. The process of claim 6, wherein said basified dispersion has a pH>9.
 8. The process of claim 6, wherein said stabilized dispersion is basified by adding a 30 wt. % aqueous solution of NH₄OH in an amount of 0.1 to 1 wt. %, based on the total weight of said stabilized dispersion.
 9. The process of claim 6, wherein said stabilized dispersion is basified by adding a 30 wt. % aqueous solution of NH₄OH in an amount of 0.1 to 0.5 wt. %, based on the total weight of said stabilized dispersion.
 10. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <400 ppm.
 11. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <300 ppm.
 12. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <200 ppm.
 13. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <100 ppm.
 14. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <50 ppm.
 15. The process of claim 1, wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <30 ppm.
 16. The process of claim 1, wherein said PTFE contains at least one comonomer selected from the group consisting of hexafluoropropene, perfluoro-(n-propyl-vinyl)-ether, perfluorohexyl vinylether, chloro-trifluoroehtylene, and mixtures thereof.
 17. The process of claim 1, wherein said PTFE is a homopolymer of tetrafluoroethylene.
 18. The process of claim 1, wherein said PTFE is a homopolymer of TFE with a SSG of 1.30 to 2.30.
 19. The process of claim 1, wherein said PTFE has a SSG of 1.90 to 2.30.
 20. The process of claim 1, wherein said starting aqueous dispersion of PTFE comprises at least one fluorinated surfactant selected from the group consisting of straight chain or branched chain C₄-C₁₂-perfluorocarboxylic acid salts, straight chain or branched chain C₄-C₁₂-hydrofluorocarboxylic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane sulfonic acid salts, and straight chain or branched chain C₄-C₁₂-hydrofluoroalkane sulfonic acid salts, straight chain or branched chain C₄-C₁₂-perfluoroalkane phosphonic acid salts, and straight chain or branched chain C₄-C₁₂-hydrofluoroalkane phosphonic acid salts and mixtures thereof
 21. The process of claim 1, wherein said starting aqueous dispersion of PTFE comprises ammonium perfluorooctanoate.
 22. The process of claim 1, wherein said starting aqueous dispersion of PTFE comprises a homopolymer of TFE in an amount of between 5 and 40 wt. %, based on the total weight of said starting dispersion.
 23. The process of claim 1, wherein said starting aqueous dispersion of PTFE comprises a homopolymer of TFE in an amount of 30 to 35 wt. %, based on the total weight of said starting dispersion.
 24. The process of claim 1, wherein said starting dispersion of PTFE comprises PTFE having a primary particle size between 0.1 to 0.5 microns.
 25. The process of claim 1, wherein said concentrated PTFE dispersion comprises a homopolymer of TFE in an amount of between 30 and 80 wt. %, based on the total weight of said concentrated PTFE dispersion.
 26. The process of claim 1, wherein said concentrated PTFE dispersion comprises PTFE having a primary particle size between 0.1 to 0.5 microns after thermal concentration.
 27. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier.
 28. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one anionic emulsifier.
 29. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one cationic emulsifier.
 30. The process of claim 1, wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier and at least one anionic emulsifier.
 31. The process of claim 1, wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier and at least one cationic emulsifier.
 32. The process of claim 1, wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier, at least one cationic emulsifier, and at least one anionic emulsifier.
 33. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one nonionic surfactant which has a cloud point range from 15 to 90° C.
 34. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one cationic surfactant which has a cloud point range from 15 to 90° C.
 35. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one anionic surfactant which has a cloud point range from 15 to 90° C.
 36. The process of claim 1, wherein said non-fluorinated emulsifier is added to said starting aqueous dispersion of PTFE in an amount of 5 to 25 wt. %, based on the total weight of PTFE in said starting aqueous dispersion of PTFE.
 37. The process of claim 1, further comprising: (1b) adding an electrolyte to the stabilized dispersion, to obtain an electrolyte-containing dispersion, prior to said heating (2).
 38. The process of claim 1, wherein said electrolyte is a base.
 39. The process of claim 1, wherein the preferred base is ammonium hydroxide (30% aqueous solution).
 40. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one anionic emulsifier selected from the group consisting of sodium lauryl sulfate, ammonium lauryl sulfate, a carboxylic acid salt, a sulfuric acid ester salt, a sulfated polyoxyethylenated straight-chain alcohol having a structure of R(OC₂H₄)_(x)SO₄ ⁻M⁺, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group, a sulfated triglyceride oil, and mixtures thereof.
 41. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one cationic emulsifier selected from the group consisting of Ethomeen T/12, Ethomeen T/15, Ethomeen 18/15, Ethomeen 18/25, Ethomeen 18/60, a long-chain amine and a salt thereof, a synthetic C₁₂ -C₁₈ primary, secondary, or tertiary amine; a diamine and polyamine and a salt thereof having a structure of (RCONHCH₂CH₂)₂NH, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; a polyoxyethylenated (POE) long-chain amine having a structure of RN[(CH₂CH₂O)_(x)H]₂, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; a quaternized polyoxyethylenated long-chain amine having a structure of RN(CH₃)[(C₂H₄O)_(x)H]₂ ⁺Cl⁻, wherein R is a straight chain or branched chain C₄-C₂₀-alkyl group; an amine oxide, and mixtures thereof.
 42. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier has the following formula (1) or (2): R—O-A-H   (1) wherein R is a straight chain or a branched chain C₈₋₁₈-alkyl group and A is a polyoxyalkylene chain containing 5 to 20 oxyethylene groups and from 0 to 2 oxypropylene groups; R′—C₆H₄—O-A′-H   (2) where R′ is a straight chain or a branched chain C₄₋₁₂-alkyl group and A′ is a polyoxyethylene chain containing 5 to 20 oxyethylene groups.
 43. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier which has the following formula: R″—(C₂H₄O)_(m)—OH, where R″ is a primary or secondary C₁₁₋₁₈-alkyl group or is

where R₁ is a primary, secondary, or tertiary C₈₋₁₂-alkyl group, and m is a number from 8 to
 14. 44. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier selected from the group consisting of a polyoxyethylene, carboxylic esters, an anhydrosorbitol esters, a natural ethoxylated fat, a natural ethoxylated oils, a a natural ethoxylated wax, a glycol ester of a fatty acid, an alkyl polyglycoside, a carboxylic amides, a fatty acid glucamide, a polyalkylene oxide block polymer, a poly(oxyethylene-co-oxypropylene) nonionic surfactant, and mixtures thereof.
 45. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one emulsifier selected from the group consisting of Triton N-11, Synperonic OP11EO, Synperonic 91/6, Tergitol 15-S-12, Tergitol 15-S-9, Tergitol 15-S-7, Newcol 1310, Dispanol TOC, and mixtures thereof.
 46. The process of claim 1, wherein said non-fluorinated emulsifier comprises at least one emulsifier selected from the group consisting of Newcol 1308FA(90), Ethox 4031 Mod 38, Ethox 4031 Mod 64, Synperonic OP10EO, Triton X-100, and mixtures thereof.
 47. The process of claim 1, wherein said non-fluorinated emulsifier comprises Ethox4031.
 48. The process of claim 1, wherein said starting aqueous dispersion of PTFE is a raw autoclave dispersion.
 49. A concentrated aqueous dispersion of PTFE, which is prepared by the process of claim
 1. 50. The concentrated aqueous dispersion of PTFE of claim 49, which is formulated into an aqueous dispersion for coating applications such as seal, packing, fabric and metal coating.
 51. A method of forming a coated substrate, said method comprising: (a) forming a coating layer on a substrate, wherein said coating layer comprises a dispersion of PTFE, and wherein said dispersion of PTFE has been prepared from a concentrated aqueous dispersion of PTFE prepared by the process of claim
 1. 52. The method of claim 51, wherein said substrate comprises at least one material selected from the group consisting of fabric, metal, plastic, wood, and combinations thereof. 